Fuel systems capable of supplying either all gaseous fuel, all liquid fuel, or a mixture of the two at a preselected ratio have been successfully employed in gas turbine engines. In some instances it is desirable to be able to burn gaseous fuels having a range of calorific values in a gas turbine engine. It can be appreciated that in a place where a more economical fuel, such as a low calorific value fuel in a gas producing area, is available, it would be more economical to arrange the turbine engine to use only the low calorific value fuel to the maximum extent. In most gas producing areas the high calorific value fuel or natural gas can be sold at a profit to a waiting customer, however; the low calorific value fuel is a by-product with little or no market value. Therefore, it is desirable to utilize such low calorific value fuel on site as a fuel. Such fuels, however, are difficult to use in a gas turbine engine. For example, it is very difficult to start a gas turbine engine on low calorific value fuel. Furthermore, the low calorific value of the fuel will necessitate the burning of a greater mass/volume of fuel in order to achieve desired turbine inlet temperature within the turbine engine versus the mass/volume of fuel when burning high calorific value fuel.
A dual fuel system is disclosed, for instance in U.S. Pat. No. 2,637,334, to N. E. Starkey issued May 5, 1953. The Starkey patent discloses a dual fuel system where liquid fuel is directed to a manifold and then to a passage in the nozzle and a gaseous fuel is directed to a separate manifold and then to a different passage in the nozzle.
U.S. Pat. No. 2,826,038 issued to James Forrest Shannon et al on Mar. 11, 1958 discloses a gas turbine engine which utilizes a liquid fuel injected into the combustion chamber by a fuel pump and a low calorific value gaseous fuel supplied through a separate duct into the combustion chamber. Shannon et al provides means for diverting a proportion of the pressure air from the combustion chamber and means for converting the energy of the diverted air into useful work, whereby during the supply of low calorific value fuel to the combustion chamber air may be diverted from the combustion chamber in proportion corresponding in mass flow to the additional mass flow of fuel necessary to achieve the desired nozzle temperature.
The primary problems encountered by the prior art systems are firstly, the different fuels have been injected into the combustion chamber through separate passages or ports each connected to the different fuel source. The different calorific value of the fuel and corresponding different mass flow rate required to provide necessary fuel quantities require different fuel injector designs. For example, orifice sizes must be different to supply the quantities of fuel needed to achieve proper turbine inlet temperatures if proper fuel injector pressure drop, to control combustion process, is to be maintained. If a fuel injector has been designed for a high BTU fuel, unacceptably high fuel injector pressure drops will occur at the fuel injector when running on low BTU fuel. On the other hand if a fuel injector has been designed to operate on low BTU fuel, very low fuel injector pressure drops will occur when running on high BTU fuel; this could cause combustion driven oscillations resulting in damage to the gas turbine. Secondly, the art teaches that the atomization and mixing of the different fuels within the combustion chamber to provide efficient burning (fuel to air ratio and mixing) and power requirements require controls to provide varied amounts of combustion air. Thirdly, starting a gas turbine engine on low calorific value fuel is very difficult to accomplish. Fourthly, complexity problems are encountered when trying to proportion the combustion air needed to provide the appropriate fuel to air ratio for proper burning when burning the needed mass/volume flow of the relatively low calorific value fuel.
These problems as mentioned above complicate the structures, increase cost and complicate the system design used to burn fuels having a range of calorific values.
The present invention is directed to overcoming one or more of the problems as set forth above.